The main subject concerning a photoreceptor has been moved from an inorganic photoreceptor of Se, As, As—Se alloy, CdS, ZnO or the like to an organic photoreceptor producing the advantage in the environmental pollution, or easiness of production, and organic photoreceptors employing various kinds of materials are developed.
Recently, function separation type photoreceptors in which different materials take in charge of functions for charge generation and charge transport are of a main stream, and of these, a multilayer type organic photoreceptor in which a charge generation layer and a charge transport layer are laminated is widely utilized.
Further, turning to an electrophotographic process, an image forming technique of a latent image is broadly divided into analog image formation employing a halogen lamp as a light source and digital image formation employing a LED or a laser as a light source. The digital image formation technique of a latent image is being rapidly favored for a printer for hard copy with a personal computer nowadays or a conventional copier in view of easiness of an image processing and easiness of development to a complex machine.
In the case of the digital image formation, a laser, or specifically a semiconductor laser or a LED is employed as a light source at a time when image information converted into a digital electric signal is written on a photoreceptor as a latent image.
Near-infrared light having a wavelength of 780 nm or 660 nm or long wavelength light close to it tends to be used for emission wavelengths of these laser light and LED light. Thus, as an organic photoreceptor used during digital image formation, high sensitivity should be obtained with respect to the long wavelength light, and studies concerning whether or not such the property is produced in what kind of material among various substances have been done. Since a phthalocyanine pigment among them is comparatively synthesized simply, and tends to exhibit high sensitivity with respect to long wavelength light, an organic photoreceptor in which an oxytitanium phthalocyanine pigment as a phthalocyanine pigment is utilized has widely been studied, and also been practically available. Concerning the oxytitanium phthalocyanine pigment, specifically an oxytitanium phthalocyanine pigment having a maximum diffraction peak at a Bragg angle (2θ±0.2°) of 27.2° in CuKα X-ray diffraction (hereinafter, also referred to simply as an oxytitanium phthalocyanine pigment) exhibits high sensitivity, and has practically been available.
Since an electrostatic latent image is written on the circumferential surface of a photoreceptor by usually fixing a light source, and rotating the photoreceptor when writing on the photoreceptor as the electrostatic latent image, an exposure position for writing on the photoreceptor surface is displaced in the case of insufficient rotation runout accuracy of the photoreceptor, whereby image reproduction is degraded, resulting in quality degradation. Therefore, the fine line image is deteriorated. Specifically, in the case of a tandem system full-color image forming apparatus equipped with 4 photoreceptors placed together, when rotation runout via rotation of each of 4 photoreceptors is produced, “out of color registration” is generated, and evenness of the color image is deteriorated, whereby image quality is degraded. Also in the case of a monochromatic image forming apparatus equipped with a single photoreceptor, density unevenness is generated, resulting in degradation of image quality. Thus, the stable rotation of the photoreceptor is favored, and this has been studied so far. For example, known is a method of stably rotating a photoreceptor by engaging a groove portion provided on a flange with a protrusion of a drive shaft fitted with a gear for rotation (refer to Patent Document 1, for example).
Also known is a method of rotating a photoreceptor via jointing of a coupling member provided at the tip of the drive shaft with a coupling member provided on the photoreceptor drive shaft employing a compression spring (refer to Patent Document 2, for example).
However, it was confirmed that the drive shaft was loaded since the photoreceptor was rotated via communication of driving of the drive shaft on the surface to engage the drive shaft with the flange, resulting in the limited number of rotations of the photoreceptor and rotation runout generated in rotation of the photoreceptor. Therefore, properties of a high sensitivity oxytitanium phthalocyanine pigment having a maximum diffraction peak at a Bragg angle (2θ±0.2°) of 27.2° in CuKα X-ray diffraction, capable of forming a high-speed image do not max out.
From such the situation, an image forming method employing an image forming apparatus fitted with a photoreceptor exhibiting excellent image reproduction together with no rotation runout during image formation, in which an oxytitanium phthalocyanine pigment having a maximum diffraction peak at a Bragg angle (2θ±0.2°) of 27.2° in CuKα X-ray diffraction is utilized, and the photoreceptor in which an oxytitanium phthalocyanine pigment having a maximum diffraction peak at a Bragg angle (2θ±0.2°) of 27.2° in CuKα X-ray diffraction is utilized are desired to be developed.    (Patent Document 1) Japanese Patent O.P.I. Publication No. 2007-232794    (Patent Document 2) Japanese Patent O.P.I. Publication No. 2007-218403